Typography: Font and size difficult to read
I will present this issue based on the messaging app and the dialer (phone app) since these are frequently used on a mobile device. On these the default font and font sizes are such that it is impossible for me to read these without glasses. My eyesight is +1.5 on both eyes, which is quite common.
The trouble is caused by a combination of very thin characters in the font and very small font size.
Comparing my Ubports Nexus 5 to my device at work, a Samsung Galaxy S7 running Android, the screen sizes are virtually identical (the S7 being ever so slightly bigger). In the messaging view the S7 shows 8 messages, while the N5 shows 7. At the same time the fonts are larger on the S7 and the characters thicker, making them possible to read.
There will, naturally, be a limit to accessibility depending on the user's eyesight, but the default should at least aim to be comfortable for the widest possible user group.